Conventionally, there is an image forming apparatus having a function of decoloring an image formed on paper. For example, the image forming apparatus heats the paper printed with decolorable toner to a certain temperature to decolor the image on the paper. In this way, the image forming apparatus makes the paper printed with the decolorable toner reusable.
Generally, developing agent obtained by mixing the decolorable toner with magnetic carrier is used in an image forming apparatus. When the developing agent is stirred, the decolorable toner may be contaminated due to the magnetic carrier. In a case in which printing is carried out on paper with the contaminated decolorable toner, the color may be left on the paper without being decolored even if the paper is heated to the certain temperature.